The present invention relates to a surface acoustic wave filter for use in the high frequency (HF) circuit, the intermediate frequency (IF) circuit, etc. of wireless communication apparatus.
The transversal surface acoustic wave filter (SAW filter), with which the amplitude characteristic and the phase characteristic can be designed independently to each other, is suitable for use as, for example, an IF filter in CDMA mobile phone, which requires relatively broad pass band and the flatness of phase characteristic within the pass band is an important factor of the filter.
A conventional transversal SAW filter using unidirectional electrodes is described below.
FIG. 30 shows plan view of a conventional transversal SAW filter using unidirectional electrodes. Referring to FIG. 30, surface acoustic wave can be generated by providing inter digital transducer electrodes (IDT electrode) for input and output on a piezoelectric substrate 301. On the piezoelectric substrate 301, an input IDT electrode 302 and an output IDT electrode 303 are disposed with a certain specific distance between each other to form a transversal SAW filter.
In the above-configured SAW filter, the frequency characteristic is determined by means of weighting, which is conducted by thinning out electrode finger(s) of IDT electrodes 302, 303. In this way, the conventional technology implements the broad and flat filtering characteristic within pass band as well as a superior attenuation characteristic at the vicinity of the pass band region.
For use in today""s compact and lightweight mobile terminals, the IF stage SAW filters are also requested to be smaller in size. In order to provide a transversal filter with a steep attenuation characteristic at the vicinity of pass band, the input and output IDT electrodes need to undergo sufficient weighting; which naturally leads to a longer length with the IDT electrodes. This blocks downsizing of the filter. Furthermore, if it is not possible to conduct the weighting on electrode without restriction, targeted characteristic may not be realized to a full satisfaction.
U.S. Pat. No. 6,011,344 discloses an electrode structure of SAW transducer, where two electrodes are provided on a piezoelectric substrate with a xc2xd wavelength shift, and electrodes of the two SAW transducers are connected in parallel. The proposed structure is intended to improving the high input/output impedance pertinent to the conventional DART type transducers. This, however, does not bring with it any contribution to reduce size of a SAW filter itself.
Japanese Laid-open Patent No. 2000-77974 discloses a SAW filter having a first and a second channels formed on a piezoelectric substrate (a 2-channel filter). In the 2-channel filter, the first channel and the second channel have the same phase characteristic within pass band, while it is inverse outside the pass band region. Although the band characteristic of the 2-channel SAW filter is steep and the size is compact, the characteristic within pass band is not flat.
The present invention aims to offer a compact SAW filter whose pass band is broad and flat within pass band, and the attenuation property is steep at the vicinity of the pass band.
SAW filter of the present invention comprises at least two filter tracks on a piezoelectric substrate, each of which filter tracks having at least two IDT electrodes for input and output. In the SAW filter, input IDT electrode of one filter track is connected in parallel with input IDT electrode of the other filter track, while output IDT electrode of one filter track is connected in parallel with output IDT electrode of the other filter track. Furthermore, frequency values of the respective filter tracks substantially coincide at a point 3 dB lower from the peak value of the transfer function. As to the phase relationship in the transfer function of the two filter tracks, it is substantially same-phased within a pass band, while it is substantially inverse outside the pass band. Thus a SAW filter of the present invention is smaller in size, broader in the pass band and steep in the attenuation characteristic outside the pass band.